Measuring devices, which are designed for progressive tracking of a target point and a coordinate position determination of this point, can generally, in particular in conjunction with industrial surveying, be summarized under the term laser trackers. A target point can be represented in this case by a retroreflective unit (for example, a cube prism), which is targeted using an optical measurement beam of the measuring device, in particular a laser beam. The laser beam is reflected in parallel back to the measuring device, wherein the reflected beam is captured using a capture unit of the device. An emission or reception direction of the beam is ascertained in this case, for example, by means of sensors for angle measurement, which are associated with a deflection mirror or a targeting unit of the system. In addition, a distance from the measuring device to the target point is ascertained with the capture of the beam, for example, by means of runtime or phase difference measurement or by means of the Fizeau principle.
Laser trackers according to the prior art can additionally be embodied having an optical image capture unit having a two-dimensional, light-sensitive array, for example, a CCD or CID camera or a camera based on a CMOS array, or having a pixel array sensor and having an image processing unit. The laser tracker and the camera can be installed one on top of another in this case, in particular in such a manner that the positions thereof in relation to one another are not variable. The camera is, for example, rotatable together with the laser tracker about its essentially perpendicular axis, but is pivotable up-and-down independently of the laser tracker and is therefore arranged separately from the optics system of the laser beam in particular. Furthermore, the camera—for example, in dependence on the respective application—can be embodied as pivotable about only one axis. In alternative embodiments, the camera can be installed in an integrated construction together with the laser optic in a shared housing.
With the capture and analysis of an image—by means of image capture and image processing unit—of a so-called measuring aid instrument having markings, the relative locations of which to one another are known, an orientation of an object (for example, a probe), which is arranged on the measuring aid instrument, in space can be concluded. Together with the determined spatial position of the target point, furthermore the position and orientation of the object in space can be precisely determined absolutely and/or in relation to the laser tracker.
The object, the position and orientation of which is surveyed using the mentioned measuring device, therefore does not have to be a measuring probe itself, for example, but rather can be the measuring aid. It is brought into a position, as part of the measurement system for the surveying, which is mechanically defined in relation to the target object or is determinable during the surveying, wherein the position and optionally the orientation of the measuring probe, for example, can be concluded via its surveyed position and orientation.
Such measuring aid instruments can be embodied by so-called scanning tools, which are positioned having the contact point thereof on a point of the target object. The scanning tool comprises markings, for example, light spots, and a reflector, which represents a target point on the scanning tool and can be targeted using the laser beam of the tracker, wherein the positions of the markings and the reflector in relation to the contact point of the scanning tool are precisely known. The measuring aid instrument can also be, in a way known to a person skilled in the art, a handheld scanner equipped for distance measurement, for example, for contactless surface surveying, wherein the direction and position of the scanner measurement beam used for the distance measurement are precisely known in relation to the light spots and reflectors which are arranged on the scanner. Such a scanner is described, for example, in EP 0 553 266.
Laser trackers of the prior art comprise at least one distance meter for distance measurement, wherein it can be designed as an interferometer, for example. Since such distance measuring units can only measure relative distance changes, so-called absolute distance meters are installed in addition to interferometers in current laser trackers. The interferometers used in this context for the distance measurement primarily use—because of the long coherence length and the measurement range thus enabled—HeNe gas lasers as light sources. The coherence length of the HeNe laser can be several hundred meters in this case, so that the ranges required in industrial metrology can be achieved using relatively simple interferometer structures. A combination of an absolute distance meter and an interferometer for distance determination using a HeNe laser is known, for example, from WO 2007/079600 A1.
In addition, in modern tracker systems, a deviation of the received measurement beam from a zero position is ascertained on a fine targeting sensor—increasingly as a standard feature. By means of this measurable deviation, a position difference between the center of a retroreflector and the point of incidence of the laser beam on the reflector can be determined and the alignment of the laser beam can be corrected or tracked as a function of this deviation such that the deviation on the fine targeting sensor is decreased, in particular is “zero”, and therefore the beam is aligned in the direction of the reflector center. By way of the tracking of the laser beam alignment, progressive target tracking (tracking) of the target point can be performed and the distance and position of the target point can be progressively determined in relation to the measuring device. The tracking can be implemented in this case by means of an alignment change of the deflection mirror, which is movable by a motor, provided for deflecting the laser beam and/or by a pivot of the targeting unit, which comprises the beam-guiding laser optic.
For the determination of the orientation of the measuring aid, a capture direction of the camera is progressively aligned so that an image can be captured in the direction of the tracking beam of the laser tracker. The camera can furthermore have a zoom function, wherein an enlargement step can be set in dependence on the determined distance between laser tracker and target point or measuring aid. Using these two adaptation functions (alignment and enlargement), the camera can therefore progressively capture an image in which the measuring aid and in particular the light spots of the measuring aid are imaged. A two-dimensional image, which can be electronically analyzed, of a spatial arrangement of light spots thus results.
An image processing unit is provided for analyzing the image. An identification of the imaged light spots, a determination of the focal points of the imaged light spots, and a determination of the image coordinates of these focal points can be performed thereby, from which, for example, spatial angles between the optical axis of the sensor, in particular the capture direction, and the direction from the sensor to the respective light spots can be calculated.
Such a coordinate measuring device having a laser tracker and an image capture unit for the determination of position and orientation of objects in space, on which light spots and reflectors are arranged, is described, for example, in U.S. Pat. No. 5,973,788.
During the use of such coordinate measuring devices, at least three light spots, which can be registered by the image capture unit, and at least one reflector, which reflects the measurement beam of the laser tracker, are arranged on the object, the position and orientation of which is to be determined, in positions known in relation to the object. The light spots to be registered by the image capture unit can be active light sources (for example, light-emitting diodes) or reflectors to be illuminated, wherein the light spots are equipped or arranged such that they can be unambiguously differentiated from one another.
A laser-based coordinate measuring device of the type in question is disclosed in WO 2007/079600 A1, in which a light exit and light receiving optics system of the distance measuring device, a measurement camera, and an overview camera are arranged on a shared element, which is rotatable with respect to at least two axes, and a laser beam is coupled by means of an optical waveguide from a laser module, which is attached outside the beam deflection unit, into the distance measuring device.
Laser-based coordinate measuring devices of the type in question generally have an electronic inclination sensor, which is integrated in a stationary base or a support of the device, which is rotatable about the standing axis.
These inclination sensors are capable of measuring an angle in relation to the vertical direction. They do not only assume the function of a height compensator for the vertical circle, but rather also monitor the inclination of the standing axis in both directions for horizontal leveling and monitoring of the stability during the measurement. By means of angle encoders on the axes of the device, an inclination of the beam deflection unit can thus also be ascertained indirectly, whereby a base coordinate system of the measurement system can be aligned in relation to the vertical direction.
In particular in the case of applications which require high-precision measurements, an inclination sensor provided in the base sometimes cannot provide sufficiently precise specifications about the present inclination of the beam deflection unit. Thus, even in the case of minor deviations between stationary base and rotatable support from a standard alignment, exact values can no longer be supplied. An inclination sensor integrated in the movable support can no longer deliver reliable specifications on the vertical direction during rotational movements of the support, in contrast, so that, for example, a lateral inclination occurring during the measurement or wobbling of the device as a result of uneven ground remains unrecognized, whereby the measurement results are corrupted.